Maybe it is because the Bavli is much easier to learn? This was for the past millennia, but with the recent advent of the Yedid Nefesh translation of Yerushalmi into Ivrit by Rav Yechiel Avrohom Bar Lev shlita, & Steinsaltz on Peoh, and even more recently with Artscroll bringing out Yerushalmi tractate by tractate, the Din for someone today not learning Yerushalmi, shall be far more severe than for any past generation.
I have already learned Shevi’is twice, once each past Shemita, as well as Berochos & Peoh, and IY’H more to come.